I would recommend PostgreSQL for a beginner as it has far fewer surprises than MySQL.
Here are some of the things that people run into with MySQL:
- http://blog.amber.org/2005/09/27/least-common-denominator/
- http://arstechnica.com/civis/viewtopic.php?f=20&t=92525
- http://exortech.com/blog/2009/11/30/weekly-release-53-environment-bug-bites/
Meanwhile, PostgreSQL does exactly what you expect it to do in most situations, and usually has a very good reason when it does something unexpected.